This invention relates to ferroresonant power supply circuits and in particular to those with closed feedback loops.
Ferroresonant transformers presently find widespread use in line voltage regulators and DC power supplies. Ferroresonant devices utilize transformer saturation to obtain output voltage regulation over input line voltage changes. Secondary saturation insures that the secondary voltage cannot increase beyond a certain value, independent of variations in primary (input) voltage.
When the voltage level of the AC input to the ferroresonant power supply reaches a certain voltage level, the core under the secondary winding saturates in each AC half cycle. At the point of saturation, the impedence of the saturating transformer (reactor) drops abruptly and capacitive current flows through the low impedence, thus carrying the capacitor charge to the opposite plate of the capacitor. As the capacitor discharges, the saturation flux density in the secondary cannot be sustained, and the reactor snaps out of saturation. At this point almost no capacitive current flows. A new half cycle begins when sufficient volt-seconds are again applied to the reactor to initiate saturation. The energy stored in the capacitor during each half cycle insures that secondary saturation will occur over a wide range of possible loads. Further increases in line voltage beyond the saturation cut-in point are absorbed across the linear inductor. Therefore, the secondary voltage remains constant over changes in line voltage. A more detailed description of ferroresonance and its application to regulated power supplies can be found in Transformer and Inductor Design Handbook, William T. McLyman, Marcel Dekker, Inc. (1978) which is incorporated by reference, as if fully set forth herein.
Standard ferroresonant power supplies utilize core saturation to achieve line regulation. However, since the core is the regulating element, it cannot regulate against influences external to the core such as frequency changes and losses in external wiring. Ferroresonant power supplies can be improved to regulate against frequency and load changes by adding a feedback control circuit to the ferroresonant transformer. According to one such improvement, the transformer core is never allowed to saturate. Instead, an AC switch connects an inductor in parallel with the AC capacitor to provide a low impedence discharge path for the capacitor. By closing the AC switch for a fraction of each half cycle, a ferroresonant discharge is simulated and the output voltage in the secondary winding can be varied as necessary with a feedback loop. This arrangement is commonly referred to as a controlled ferroresonant power supply. This improvement, however, results in increased loop gain and potentially unstable conditions at certain frequencies. Input AC line transients and rapidly changing load conditions can easily trigger sustained oscillations.
Prior art teaches that loading down the output of the ferroresonant power supply enhances stability by reducing the likelihood of sustained oscillation. Such a solution to the instability problem is unsatisfactory since part of the total available output power of the power supply must be dissipated to provide stability. As much as 10% of the available output may be required to insure the power supply will not oscillate. When this reduction of available output power has been found unacceptable the alternative in the prior art has been to monitor the output voltage from the ferroresonant power supply with a control circuit to sense output instability. When oscillations occur, the control circuit may "crowbar" or shutdown the power supply. This solution is also inadequate since it may result in the untimely shutdown of the power supply. Moreover, crowbarring or shutting down the ferroresonant power supply is not a solution to the problem, but only a safeguard mechanism to protect other equipment from damage caused by the instability of the ferroresonant power supply. Therefore, there is a need for a controlled ferroresonant power supply which can be operated stably over a no load to full load range without requiring the dissipation of power supply output power or the shutting down of the power supply.
An object of this invention is to provide a new and improved construction of a controlled ferroresonant power supply which maintains operational stability over input line transients and rapid variations in output load.
A further object of this invention is to provide a controlled ferroresonant power supply which permits stable operation with no external load.